It is well known that extremely high voltages can develop in the vicinity of an integrated circuit due to the build-up of static charge. A high potential may be generated to an input or output buffer of the integrated circuit, which may occur by a person simply touching a package pin that is in electrical contact with an input or output buffer of the integrated circuit. Upon electrostatic discharge, a high current is produced at the package nodes of the integrated circuit. Due to the potential of destroying the entire integrated circuit, ElectroStatic Discharge (ESD) is a serious problem for semiconductor devices.
One difficulty in designing ESD protection circuits is based on the demanding performance requirements that must be met. The characteristics of most semiconductor devices are such that they can be damaged even when subjected to very short duration transient voltages. Therefore, over voltage protection devices such as fuses and circuit breakers that operate on a thermal principal ordinarily respond much too slowly and usually cannot be relied upon to adequately protect semiconductor devices.
For this reason, it is rapidly becoming a known practice to incorporate an ESD protection system in the circuitry itself. Traditionally, a diode string is used. FIG. 1 is a schematic of a typical diode string 110 that includes four diodes 1141-1144 (any one of which can be referred to as diode 114) formed in a silicon substrate (shown in FIG. 2). The circuit 100 also includes a reverse diode 112. Each diode 114 has a voltage called the threshold voltage or turn on voltage. When a voltage applied in the forward direction exceeds the turn on voltage of a diode, the diode changes to an “on” or conducting state. In the case of ESD protection, static charges are discharged through the diode string 110, and the voltage that can be applied on any one diode 114 is limited even though the current may be high. The diode string 110 also has a turn on voltage at which all diodes 114 change to a conducting state and the diode string 110 starts conducting. When the diode string 110 is turned on, static charges are discharged through the diode string, and the voltage between Vdd and Vss is brought down. During a positive voltage transient, the diode string 110 turns on and sinks ESD current, and during a negative voltage transient, the reverse diode 112 turns on and sinks the ESD current.